1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly to a photomask for overcoming a limitation on the resolution of a stepper, and a method for forming micro patterns using the photomask.
2. Description of the Related Art
For light exposure machines such as steppers, those using a light source of a shorter wavelength have conventionally been used in order to obtain a reduced pattern size. For instance, light sources of G-line (xcex=435 nm) or I-line (xcex=365 nm) have mainly been used. Recently, light sources of KrF (xcex=248 nm) or ArF (xcex=193 nm) have been proposed. Also, a light exposure process using X-rays has been proposed.
However, there is a limitation in obtaining a desired reduction in pattern size, only with a light source of a short wavelength. In order to solve such a problem, accordingly, there have also been proposed a modified illumination process and a light exposure process using two sheets of photomasks. The light exposure process using two sheets of photomasks is disclosed in Japanese Patent No. 58-127325 (published on Jul. 29, 1983) and U.S. Pat. No. 5,503,959 (issued on Apr. 2, 1996).
Hereinafter, a typical photomask, a double photomask consisting of two photomask sheets and light exposure processes respectively using those photomasks will be described in conjunction with FIGS. 1 and 2.
FIG. 1 is a schematic view illustrating a typical photomask having chromium patterns of a high density, and an intensity distribution of light exhibited in a light exposure process using the photomask. FIG. 2 is a schematic view illustrating a double photomask consisting of two photomask sheets each having a chromium pattern of low density, and an intensity distribution of light exhibited in a light exposure process using the double photomask. In FIGS. 1 and 2, the reference numeral 1 denotes a quartz substrate, the reference numbers 2, 12a, and 12b denote chromium patterns, respectively, the reference numerals 3, 13a, 13b denote spaces, respectively, and the reference numerals 10, 20a, and 20b denote photomasks, respectively.
In the case of the typical photomask 10, its fabrication is difficult because the chromium patterns 2 are formed in a high density on the quartz substrate 1, as shown in FIG. 1. Where a light exposure is conducted using this photomask 10, light passing through the space 3 may be subjected to a severe diffraction because the space 3 has a small width d1. For this reason, it is difficult to form micro line/space patterns.
Meanwhile, in the case of the double photomask consisting of two photomasks 20a and 20b, as shown in FIG. 2, its fabrication is easy, as compared to the typical photomask 10. This is because the chromium patterns 12a and 12b of the photomasks 20a and 20b have low density.
In the case of the double photomask, however, light passing through the spaces 13a and 13b may also be subjected to a severe diffraction because the spaces 13a and 13b have the same width, d2, as the width d1 of the space 3 in the typical photomask 10. For this reason, it is difficult to form micro line/space patterns even though the photomasks 20a and 20b are used.
For instance, where a light exposure is conducted using the above mentioned two photomask sheets in order to fabricate a highly dense memory device such as a 256-Mega DRAM or a 1-Giga DRAM, the respective pattern shapes of the chromium patterns 12a and 12b are insufficiently transferred to a photoresist film, formed therebeneath, due to a severe diffraction phenomenon of light. As a result, it is impossible to obtain micro line/space patterns.
Furthermore, although typical light exposure processes are conducted using a positive photoresist film, the light exposure process using the above mentioned two photomask sheets is conducted using a negative photoresist film. Such a use of the negative photoresist film results in a requirement to construct a new process line. For this reason, a separate management is required.
Therefore, an object of the invention is to provide a photomask capable of solving problems resulting from a severe diffraction phenomenon of light.
Another object of the invention is to provide a method for forming micro patterns of a semiconductor device using the photomask adapted to the above mentioned object of the present invention.
In accordance with one aspect, the present invention provides a photomask comprising: a first photomask consisting of a first quartz substrate and first chromium patterns formed on the first quartz substrate; and a second photomask consisting of a second quartz substrate and second chromium patterns formed on the second quartz substrate, wherein chromium patterns divided into two groups respectively consisting of the first chromium patterns and the second chromium patterns in such a fashion that a space defined between adjacent ones of the first or second chromium patterns is more than a space defined between adjacent ones of the first and second chromium patterns so as to avoid a severe diffraction of light passing between adjacent ones of the chromium patterns.
In accordance with one aspect, the present invention provides a method for forming micro patterns of a semiconductor device using a photomask including chromium patterns divided into two groups in such a fashion that the chromium patterns in one of the two chromium pattern groups alternate, one by one, with the chromium patterns in the other chromium pattern group, the chromium patterns being formed on two quartz substrates for the two chromium pattern groups, respectively, to prepare, for the photomask, two separate photomasks each having an increased space defined between adjacent chromium patterns thereof so as to avoid a severe diffraction of light passing between the adjacent chromium patterns, comprising the steps of: preparing a silicon substrate sequentially formed with an etch target layer, an etch barrier layer, and a first photoresist film over an upper surface thereof; exposing the first photoresist film to light by use of a first one of the photomasks, and subjecting the light-exposed first photoresist film to a development process, thereby forming first photoresist film pattern; etching the etch barrier layer, by use of the first photoresist film patterns as a mask, to partially expose the etch target layer; coating a second photoresist film over the resulting structure; exposing the second photoresist film to light by use of a second one of the photomasks, and subjecting the light-exposed second photoresist film to a development process, thereby forming second photoresist film patterns on portions of the etch target layer exposed between adjacent etched portions of the etch barrier layer, respectively; etching the etch target layer by use of the second photoresist film patterns and the etch barrier layer as a mask; and removing the remaining etch barrier layer and the second photoresist film pattern.
In accordance with still another aspect, the present invention provides a method for forming micro patterns of a semiconductor device using a photomask including chromium patterns divided into two groups in such a fashion that the chromium patterns in one of the two chromium pattern groups alternate, one by one, with the chromium patterns in the other chromium pattern group, the chromium patterns being formed on two quartz substrates for the two chromium pattern groups, respectively, to prepare, for the photomask, two separate photomasks each having an increased space defined between adjacent chromium patterns thereof so as to avoid a severe diffraction of light passing between the adjacent chromium patterns, comprising the steps of: preparing a silicon substrate sequentially formed with an etch target layer, a first photoresist film, an etch barrier layer, and a second photoresist film over an upper surface thereof; exposing the second photoresist film to light by use of a first one of the photomasks, and subjecting the light-exposed second photoresist film to a development process, thereby forming second photoresist film patterns; etching exposed portions of the etch barrier layer to a desired depth by use of the second photoresist film patterns as a mask in such a fashion that the etch barrier layer has portions of different thicknesses; coating a third photoresist film over the resulting structure; exposing the third photoresist film to light by use of a second one of the photomasks, and subjecting the light-exposed third photoresist film to a development process, thereby forming third photoresist film patterns on portions of the etch barrier layer exposed between adjacent ones of the second photoresist film patterns, respectively; etching exposed portions of the etch barrier layer by use of the second and third photoresist film patterns as a mask; etching the first photoresist film by use of a second and third photoresist film patterns as a mask; removing the second and third photoresist film patterns; etching the etch target layer by use of the etch barrier layer and the first photoresist film patterns as a mask; and removing the remaining etch barrier layer and the first photoresist film pattern.